This invention relates to a ceramic multilayer wiring board, in particular a ceramic wiring board using copper as a via interconnection, to a ceramic multilayer wiring board having a suitable post-sintering via microstructure, and to a copper paste for obtaining this microstructure.
Ceramic wiring boards having a multilayer structure are used in electrical devices where modular wiring substrates are required for high integration and high-speed processing, due to the need for making fine interconnections. Copper is the material of choice for these interconnections due to its low specific resistance.
As substrate used as a support for interconnections, an inorganic material having glass as its principal component is used as the glass can be sintered at the same time as the copper of the interconnections. A borosilicate glass suitable for substrates is described in detail in Japanese Patent Laid-Open No. Hei 8-333157. Fillers which may be added are disclosed in Japanese Patent Laid-Open No. Hei 8-18232.
Here, the method of manufacturing the substrate will be briefly described.
Generally, the inorganic material is supplied in the form substrate is manufactured by the well-known green sheet method. This method consists of the following steps.
(1) Making a slurry of the powdered inorganic material using an organic binder and a solvent.
(2) Forming this slurry into the shape of a sheet.
(3) Opening vias (through holes) in the sheet.
(4) Embedding an interconnection paste in the vias.
(5) Forming an interconnection or other pattern on the sheet surface with the interconnection paste.
(6) Laminating these sheets with interconnection patterns together under pressure.
(7) Heat treating the resulting laminate.
In the above-mentioned heat treatment process, the organic binder in the laminate and the organic substance in the interconnection paste decompose and are thus eliminated. At the same time, the inorganic material in the laminate which is in a powdered state of aggregation and the conducting metal in the conducting paste are sintered and become finer.
However, if the organic binder remains in the sintered compact, it will be converted to graphite, and the quality of the substrate and wiring after sintering will deteriorate. For this reason, sufficient binder removal time is generally allowed in the sintering step, followed by a sintering period which has the main purpose of increasing the fineness.
This classical type of heat treatment profile is disclosed in Japanese Patent Laid-Open No. Hei 8-18232, etc. The binder is removed in an atmosphere at about 800.degree. C. for 15 hours, and the product is kept in an atmosphere at about 1000.degree. C. for 2 hours for sintering. Water vapor etc. is usually added to the processing atmosphere during the above-mentioned binder removal.
However, when copper is used for the metallic material of the conductor, although sintering of the copper takes place starting from approximately 600.degree. C., sintering of the glass ceramics itself begins at a higher temperature. This difference of sintering start times may causes serious problems in the substrate, particularly in the conductor or at the interface between the conductor and the ceramics, so in the case of copper paste, an attempt is often made to adjust the sintering start temperature of the ceramics.
As an example, a copper paste mixed with alumina of particle size 0.1 .mu.m to 1 .mu.m is disclosed by Japanese Patent Publication 2584911. Also, a copper paste comprising copper oxide and glass frit is disclosed by Japanese Patent Laid-Open No. Hei 8-279666.